A digital video signal may be processed to form a bit stream having a reduced bit rate. Such processing for bit rate reduction may be implemented according to an MPEG compression method, and may be formatted for example, with a structure, such as that employed in a digital satellite system, for example DSS™ or the proposed Grand Alliance or GA terrestrial system.
A consumer video recorder may be adapted to provide recording capabilities for both analog signals and digitally encoded signals processed for example, for compatibility with an MPEG standard and having for example GA or DSS™ signal formats. The adapted consumer video recorder may be considered as two electronic systems within one box, sharing a recording mechanism, servo and control systems. A conventional analog recording method may be employed where an analog luminance signal component frequency modulates an RF carrier and an analog chrominance signal component amplitude modulates a second RF carrier. When reproduced, the two modulated signals are demodulated, combined and coupled for video display.
Digital recording may be achieved by employing the consumer video recorder as a digital bit stream recorder/player. The MPEG compatible bit stream, for example transport packets, may be recorded by reformatting and processing to facilitate recording and reproduction. In a playback mode the recording processes are effectively reversed, and the bit stream is restored to have the original MPEG format and time relationships.
The material cost of a consumer digital recorder may be minimized by the omission of MPEG processing or decoding. Thus, the recorder records and replays the MPEG packet stream without changes or additions. Hence the consumer digital recorder functions as a bit stream recorder for program delay or time shifting usage.
The MPEG compression method employs intra coded frames, or I frames, forward predicted frames, or P frames and a bidirectionally predicted frames, or B frames. These three types of frames occur in groups known as Groups Of Pictures or GOPs. The number of frames in a GOP is user definable but may comprise, for example, 12 or 15 frames. Each GOP contains one I frame, which for transmission is abutted by a P frame which is followed by a sequence of B and P frames. Only I frame data may be decoded independently of any other frame, P frames may be predicted from a preceding I or P frame. Bi-directionally predicted frames or B frames require prediction from surrounding I or P frames.
In an analog consumer VCR, trick play features, such as picture in forward or reverse shuttle, fast or slow motion, are readily achievable, since each recorded track typically contains one field with adjacent tracks containing similar, but temporally off set images. Reproduction at speeds other than the recorded speed result in the reproducing head, or heads, crossing multiple tracks, and recovering recognizable picture segments. However, when an MPEG signal is recorded, the I, P and B frames of each GOP occupy adjacent tracks. When replayed at play or normal speed, the I frame data must be recovered first in order to permit the reconstruction of succeeding P and B frames. When the MPEG recording is replayed at a speed different to normal speed the replay heads transduce sections or segments from multiple tracks. However, these replayed segments contain parts of a GOP which, unlike the analog recording condition, no longer represent sections of discrete records from consecutive images. Instead, the transduced segments contain data resulting mainly from predicted P and B frames of the GOP. Clearly, since during trick play operation the amount of I frame data replayed progressively diminishes as the trick play speed increases, there is little possibility of decoding P and B frames from the reproduced pieces of I frame data. In addition, if the replayed I frame data is to be decoded it must comply with the syntax required by the MPEG standard. Thus, in a consumer digital recorder which does not process the MPEG stream to redistribute I frame data or form trick play data for redundant recording, reproduction at other than normal play speed may be precluded.